Bilingual Course Network Communications 网络通信 ( For Master Students in the Department of Electronic Engineering) Chapter 5 Signal Encoding Instructor: Dr Tianshuang Qiu School of the Electronic and Information Engineering Fall of 2004
1 Bilingual Course Network Communications 网络通信 (For Master Students in the Department of Electronic Engineering) Chapter 5 Signal Encoding Instructor: Dr. Tianshuang Qiu School of the Electronic and Information Engineering Fall of 2004
Data Encoding Digital data, digital signal: In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment Analog data, digital signal: conversion of analog data to digital form permits the use of modern digital transmission and switching equipment. The advantages of the digital approach were outlined in Section 3. 2 Digital data, analog signal: Some transmission media, such as optical fiber and unguided media, will only propagate analog signals Analog data, digital signal: Analog data in electric form can be transmitted as baseband signals easily and cheaply. This is done with voice transmission over voice-grade lines. One common use of modulation is to shift the bandwidth of a baseband signal to another portion of the spectrum. In this way multiple signals each at a different position on the spectrum, can share the same transmission medium. This is know as frequency-division multiplexing 2
2 Data Encoding Digital data, digital signal: In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment. Analog data, digital signal: conversion of analog data to digital form permits the use of modern digital transmission and switching equipment. The advantages of the digital approach were outlined in Section 3.2. Digital data, analog signal: Some transmission media, such as optical fiber and unguided media,will only propagate analog signals. Analog data, digital signal: Analog data in electric form can be transmitted as baseband signals easily and cheaply. This is done with voice transmission over voice-grade lines. One common use of modulation is to shift the bandwidth of a baseband signal to another portion of the spectrum. In this way multiple signals, each at a different position on the spectrum, can share the same transmission medium. This is know as frequency-division multiplexing
85-1 Digital Data, Digital Signal 0:1:0:01:1:0:0:0:1:1 NRZ-L NRZI Bipolar-AMI Pseudoternary Manchester lUyuN Differential Manchester Figure 5.2 Digital Signal Encoding Formats 3
3 §5-1 Digital Data, Digital Signal
1.几种编码形式 (1) Non Return to Zero(nrz 最简单; 在一个bit周期内电平横定 正负跳变,不回0 0一由高电平表示,1—由低电平表示 优点:易于实现,有效利用带宽; 缺点:有直流成分,缺乏同步能力(若一串1或一串0,则无法同 步)
4 1. 几种编码形式 （1）Non Return to Zero (NRZ) • 最简单； • 在一个bit周期内电平横定； • 正负跳变，不回0； • 0—由高电平表示，1—由低电平表示； • 优点：易于实现，有效利用带宽； • 缺点：有直流成分，缺乏同步能力（若一串1或一串0，则无法同 步）
(2) Nonreturn to Zero inverted(NRZⅠ不归零1制 0—在间隔的起始位置无跳变 1—在间隔的起始位置有跳变 (其他特点同NR (3) Bipolar AMI( Alternate mark inversion,交替信号反转) 0一没有信号 1—正/负电平 优点:很长的串1,不会失去同步; 信号1在正负电平之间跳变,无直流分量; 信号的带宽比较窄 脉冲交替,有利于差错检测 (4) Pseudoternary(伪三进制) ·0—正电平或负电平; 1—没有信号
5 （2）Nonreturn to Zero Inverted (NRZI, 不归零1制) • 0—在间隔的起始位置无跳变 • 1—在间隔的起始位置有跳变 （其他特点同NRZ） （3）Bipolar AMI (Alternate mark inversion, 交替信号反转) • 0—没有信号； • 1—正/负电平 • 优点：很长的串1，不会失去同步； 信号1在正负电平之间跳变，无直流分量； 信号的带宽比较窄； 脉冲交替，有利于差错检测 （4）Pseudoternary（伪三进制） • 0—正电平或负电平； • 1—没有信号
(5) Manchester(曼彻斯特码) 0—在间隔中间负向跳变; 1在间隔中间正向跳变; 优点: 同步好,每bit均有跳变,又称为自对称; 没有直流成分 有助于差错检测; 缺点:带宽是NRZ的2倍 (6)差分曼彻斯特码 ·0—在间隔的起始位置跳变,在中间有跳变; ·1—在间隔的起始位置不跳变,在中间有跳变
6 （5）Manchester（曼彻斯特码） • 0—在间隔中间负向跳变； • 1—在间隔中间正向跳变； • 优点： - 同步好，每bit均有跳变，又称为自对称； - 没有直流成分； - 有助于差错检测； • 缺点：带宽是NRZ的2倍 （6）差分曼彻斯特码 • 0—在间隔的起始位置跳变，在中间有跳变； • 1—在间隔的起始位置不跳变，在中间有跳变；
不同编码方式的频谱特性 a23d10 b1厂aca上5用2 yon n legend 21用O B8Zs HDB3 d: AMI i a Alternate mark inversion SP B8ZS Bipolar with 8 zeros substitution Freque HDB3 ligh-density bipolar--3 zeros NRZ-L NRZL-L= Nonreturn to zero level 1.0 F.NRZI 128 NRZI= Nonreturn to zero inverted R 0.8 aml pseudoternary o d s 1o sungod or 90.6 e wesTener Manchester Differential Mancheste 0.2 0.40.60.81.01.21.4161.8 2.0 Normalized frequency (/R) Figure 5.3 Spectral Density of Various Signal Encoding Schemes
7 不同编码方式的频谱特性
各种编码方法的错误率(理论值) 101 AMI, pseudoternary, 102 ASK FSK 10 NRZ, biphase PSK, QPSK 104 105 106 10 O1=2345678910112131445 (EyNo(dB) Figure 5.4 Theoretical Bit Error Rate for Various Encoding Schemes
8 各种编码方法的错误率（理论值）
不同数字信号编码速率的归一化信号跳变速率 Table 5.3 Normalized Signal Transition Rate of Various Digital Signal Encoding Rates Minimum 101010 Maximum NRZ-L 0(all Os or 1s) 1.0 1.0 NRZI 0(all 0.5 1.0(all 1s)L Bipolar-AMI 0(all Os) 1.0 1.0 Pseudoternary o(all 1s)L 1.0 1.0 Manchester 1.0(010-) 1.0 2.0(all Os or 1s) Differential Manchester 1.0 (all 1s 15 20(l0s)
9 不同数字信号编码速率的归一化信号跳变速率
2. Modulation rate(调制速率) Description: When signal encoding techniques are used, a distinction needs to be made between data rate(expressed in bits per second) and modulation rate (expressed in baud). The data rate, or bit rate, is R-1/Tb, where Tb is bit duration The modulation rate is the rate at which signal elements are generated Consider for example, Manchester encoding. The minimum size signal element is a pulse of one-half the duration of a bit interval. For a string of all binary zeros or all binary ones, a continuous stream of such pulses is generated. Hence the maximum modulation rate for manchester is d=2 /Tb. This situation is illustrated in Figure 5.5(E6), which shows the transmission of a stream of binary ls at a rate of MBps using nrzi and manchester. In general, D=R/b where D= modulation rate. baud R=data rate. bps b=number of bits per signal element
10 2. Modulation Rate（调制速率） Description: When signal encoding techniques are used, a distinction needs to be made between data rate (expressed in bits per second) and modulation rate (expressed in baud). The data rate, or bit rate, is R=1/Tb, where Tb is bit duration. The modulation rate is the rate at which signal elements are generated. Consider, for example, Manchester encoding. The minimum size signal element is a pulse of one-half the duration of a bit interval. For a string of all binary zeros or all binary ones, a continuous stream of such pulses is generated. Hence the maximum modulation rate for Manchester is D=2/Tb. This situation is illustrated in Figure 5.5（E6）, which shows the transmission of a stream of binary 1s at a rate of 1Mbps using NRZI and Manchester. In general, D=R/b. where D = modulation rate, baud R = data rate, bps b = number of bits per signal element